This invention pertains to x-ray systems and more particularly concerns a high voltage power supply for grid controlled x-ray tubes.
X-ray tubes are thermionic emission devices and sensitive to both filament temperature and high voltage applied to the tube. Accordingly, most dental x-ray systems now include some sort of feedback circuits to regulate both high voltage and filament current.
A common way to provide high voltage is to apply line voltage to a high turns-ratio transformer, the output of which provides kilovoltage directly across the anode and cathode of the x-ray tube. Because of the high turns ratio required by a direct conversion of the voltage, the high voltage transformers used in previous designs are quite large and bulky. Often there is high flux leakage in the transformer, resulting in a low coupling coefficient and a corresponding reduction of the efficiency of the transformer. Furthermore, because the secondary coil requires many windings, there is high internal resistance. Because of this high resistance, and also because of possible changes in leakage, the voltage across the tube is not constant but varies according to tube current, as well as line voltage. Tube current, however, is highly sensitive to filament temperature and, therefore, accurate means of regulating filament current have been required, including means to pre-heat the filament before the high voltage is applied.
It will be seen that my invention offers substantial improvement over many pre-existing used circuits by meeting the objectives of reducing transformer size, cost and weight, providing an initially constant high voltge, providing compensation for line voltage fluctuations, and less sensitivity to tube current.